Predicting that the river would continue to rise through the end of the month, Da Costa Farias said the swift evacuation of residents in the worst-affected areas had ensured there were no fatalities.

Last month, authorities declared a state of emergency in the regional capital Porto Velho after the rising waters left many roads submerged.

About 4,000 were displaced in Acre state, bordering Peru.

President Dilma Rousseff, who flew over the area last weekend, was forced to interrupt a break after torrential rain hit the south of the country, leaving some 60 people dead and tens of thousands of people homeless.

Other Brazilian regions have in contrast suffered severe drought this year, and the state government of business hub Sao Paulo moved to rule out water rationing.

"There will be no rationing in municipalities covered by (state water firm) Sabesp," Governor Geraldo Alckmin said, insisting reserve supplies would suffice.

Sabesp manages supplies for some 70 per cent of the state's 41.2 million inhabitants.

Media had speculated in recent days that rationing might be required as Brazil prepares for an influx of tourists, with Sao Paulo due to host the opening match of the World Cup on June 12.